In a conventional type speed change gear drive for a transmission of an engine mounted in a vehicle, the engagement of a group of transmission gears of a transmission is changed and a shift to reduction gear ratio corresponding to predetermined speed is performed by driving a shift drum on the periphery of which cam grooves are formed and suitably changing a position of a shift fork along patterns of the cam grooves. The shift drum is driven by turning a change spindle linked with, among other things, a change pedal. The shift drum is supported by both side walls of a transmission case so that the shift drum can be rotated, the change spindle arranged in parallel with the shift drum is supported by both side walls of the transmission case and its both ends are protruded outside the transmission case. The change pedal and others are coupled to one end of the change spindle and the change arm is fixed to the other end. At the end on the side of the change arm of the shift drum, driving parts including the change arm for rotating the shift drum itself and angle regulating parts for regulating an angle of rotation are collectively arranged. For an example, refer to Japanese Patent 6-117533.
In the above-mentioned known speed change gear drive for a transmission, a shift fork rod that supports the shift fork is arranged in parallel with the shift drum and the change spindle and both of its ends are supported by side walls of the transmission case. Typically, the shift drum, the change spindle and the shift fork rod are arranged in parallel, however, to further miniaturize an engine, alternative layouts are desired.